Optical instruments for determining the index of refraction (n) of a substance have long been known under the designation refractometer. In these classical optical apparatus, the determination of the refractive index preferably occurs by measuring the critical angle of the total reflection by utilizing a measurement prism having a known refractive index.
So-called interference refractometers are also known wherein the determination of the refractive index occurs with the aid of interferometric means. In the known apparatus, the detection of the measured value occurs visually or also by means of complex electronics.
All of these known apparatus comprise individual optical parts of high quality and require much space. The highest precision which is required for the manufacture of the optical and precision-mechanical components as well as the electronic evaluation and automatic apparatus make the cost thereof high.
Accordingly, the foregoing has resulted in a desire amongst engineers and technicians for substantially smaller, simpler and more portable apparatus at a lower cost.
An interesting and promising path toward miniaturization of optical instruments was presented already in 1969 in the technical paper entitled "Integrated Optics: an introduction" by Stewart E. Miller, the Bell System Technical Journal, Volume 48, No. 7, pages 2059 to 2069, September 1969. The suggestions made in this paper with regard to the assembly and utilization of integrated optical components were however not realizable because of technical reasons.
Only after an intensive technological development in the area of materials and mask technology could substantial advances be obtained in the realization of integrated optical components during the past several years. Suitable substrate materials made of glass, crystalline substances or transparent plastics were developed in which wave guides of suitable configuration could be integrated by means of ion diffusion, ion implantation or by means of applying organic or metal-organic layers.
As a consequence of the foregoing, integrated-optical components have been developed which have taken over the following tasks in transmission systems: modulation, switching, interconnecting channels, branching and the like. Even apparatus for measuring physical quantities have become known wherein integrated-optical components are used as sensors.
In the publication "Schott-Information", Volume 3, page 29, FIG. 3, (1987), an integrated-optical hydrogen sensor is shown. An integrated-optical temperature sensor is described in East German Pat. No. 249,772 wherein the sensor has an interferometer configuration wherein temperature-dependent changes of the phase difference between the light running through both interferometer branches is utilized for measuring temperature.